Petroleum is found in subterranean formations or reservoirs in which it has accumulated, and recovery is usually accomplished initially by so called primary production comprising pumping or permitting the petroleum to flow to the surface of the earth through wells drilled into and completed in the subterranean petroleum formation. Petroleum can be recovered from subterranean formations only if certain conditions exist. There must be an adequate concentration of petroleum in the formation and there must be adequate permeability or interconnected flow channels throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the subterranean, petroleum-containing formation has natural energy present in the form of an underlying active water drive, solution gas, or a high pressure gas cap above the petroleum within the formation, this natural energy is utilized to recover petroleum. In the primary phase of petroleum recovery, petroleum flows to wells drilled into and completed in the formation, the petroleum being displaced toward the wells by naturally-occurring energy in the formation. When the natural energy source is depleted, or in the instance of those formations which do not originally contain sufficient natural energy to permit primary petroleum recovery operation, some form of supplemental recovery process must be applied to the reservoir. Supplemental recovery is frequently referred to in the literature as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
Petroleum recovery operations involving the injection of water into the formation for the purpose of displacing petroleum toward the producing wells, commonly referred to as water flooding or secondary recovery, is the most economically and widely practiced form of supplemental recovery. Water does not displace petroleum efficiently, however, since water and oil are immiscible and the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this weakness of water flooding and many additives have been described in the prior art for decreasing the interfacial tension between the injection water and the formation petroleum. Petroleum sulfonates have been disclosed in many references for use in surfactant water-flooding oil recovery operations, but petroleum sulfonates have limitations with respect to formation water salinity, hardness, and other factors which restrict the usefulness thereof. U.S. Pat. No. 3,811,504 describes a three component surfactant system comprising an anionic surfactant such as alkyl or alkylaryl sulfonate plus a nonionic surfactant such as polyethoxylated alkyl phenol and a alkylpolyethoxy sulfate. U.S. Pat. No. 3,811,505 describes a dual surfactant system employing an anionic surfactant such as an alkyl or alkylaryl sulfonate or phosphate, plus a nonionic surfactant such as a polyethoxylated alkyl phenol or polyethoxylated aliphatic alcohol. U.S. Pat. No. 3,811,507 describes an oil recovery method using a dual surfactant system comprising anionic surfactants such as a linear alkyl or linear alkylaryl sulfonate, plus a sulfated polyethoxylated aliphatic alcohol. These systems permit the use of surfactant flooding processes in formations containing water containing from 500 to 12,000 parts per million polyvalent ions, such as calcium and magnesium. U.S. Pat. No. 4,018,278 relates to a surfactant oil recovery process suitable for use in high temperature formations containing very high salinity formation water, employing a single surfactant-containing fluid, the surfactant being an alkylpolyethoxy sulfonate or an alkylarylpolyethoxy sulfonate in a saline fluid whose salinity is about equal to the salinity of the formation water. U.S. Pat. Nos. 3,827,497 and 3,890,239 relate to a surfactant fluid and an oil recovery processes employing a fluid containing an organic sulfonate, a sulfated or sulfonated oxyalkylated alcohol and a polyalkylene glycol alkyl ether. U.S. Pat. No. 3,333,634 describes an oil recovery method employing a shear-thickening surfactant fluid containing an alkylaryloxypoly(ethoxy)ethanol and a dialkyl sulfosuccinate. U.S. Pat. Nos. 3,500,923 and 3,508,612 describe oil recovery fluids and methods employing alkylpolyethoxy sulfates and alkylarylpolyethoxy sulfates.
The surfactant fluid to be injected into a subterranean, petroleum-containing formation for petroleum recovery purposes are routinely injected into a relatively low permeability formation, and so the injectivity of the fluid is of critical importance. The presence of dispersed particulate matter which may cause plugging of the subterranean formation will lead to serious problems, since the fluid must be injected into the formation over long periods of time, and plugging is especially significant if it occurs in the portion of the petroleum-containing formation immediately adjacent to the injection wells. Injectivity is most critical at the formation face at the wellbase since this is most likely to have suffered permeability loss due to the drilling and completion operation, and the area through which the fluid passes is smallest near the wellbase.
The fluid must be present in the relatively hostile environment of the subterranean, petroleum-containing formation for long periods of time, in the order of many months to several years, as it passes through the portion of the formation to be swept by the injected fluid between the injection well and production well. The fluid must, therefore, remain relatively stable, both chemically and physically, over the period of time it will be present in the formation. Phase instability is frequently encountered in the use of the more complex surfactants which are used for recovering oil form formations containing relatively high salinity water. Once phase instability has occurred, the oil recovery effectiveness of the fluid is destroyed, and so it is of paramount importance to ensure that the fluid remains phase stable for the period of time it remains in the formation. Finally, the fluid will effectively displace oil from the microscopic pores, including the smaller flow channels and dead-end pore spaces of the formation, only if the interfacial tension, i.e., the surface tension between the aqueous surfactant-containing phase and the formation petroleum can be maintained at a very low level. Accordingly, it can be appreciated that attainment of the lowest possible interfacial tension is of critical importance to the successful application of the surfactant fluid in oil recovery operations.
It is, therefore, an object of the present invention, to provide a method for treating the surfactant fluid to increase the ease with which it is injected into a relatively low permeability oil formation without encountering injectivity or plugging problems.
It is a further object of the present invention to provide a method for treating the fluid to increase the stability of the surfactant fluid, thereby increasing the time which the surfactant fluid will remain in the necessary single phase condition in the formation without separating into multiple phases which degrades the effectiveness of the surfactant fluid for surfactant flooding purposes.
It is still another object of the present invention to provide a method for treating a surfactant fluid to be used for surfactant flooding oil recovery purposes, so as to increase the effectiveness of the fluid for reducing interfacial tension between the surfactant fluid and formation petroleum, thereby increasing the microscopic displacment efficiency of the surfactant fluid.
At least certain of the foregoing objects will be met by at least some of the preferred embodiments of my invention to be described more fully hereinafter below.